OK, I am embarassed to admit that I am having a brain freeze and cannot see this answer. I know I was taught this once. 
I have two points. X1, Y1 and X2, Y2.
I have the X value of the third point. X3. It falls between X1 and X2.
How do I figure out Y3?
Are the points all supposed to fall on a straight line?
Assuming that all of the points fall on a straight line:
The slope of this line, m, is (y2 - y1)/(x2 - x1)
The y-intercept of the line, b, is y2 - m * x2.
The equation for this line is:
y = m * x + b
So y3 = m * x3 + b
Thanks. Yes, all on a straight line - my apologies for asking the question so poorly and my thanks for helping with the answer!